The present invention relates to an aperture control apparatus of a programming shutter .
The control operation of the programming shutter using shutter blades having the function of diaphragm blades is now described with reference to FIG. 6.
FIG. 6 shows a known opening characteristic curve of the programming shutter of this type, in which the abscissa represents time and the ordinate represents the aperture value.
At a time t.sub.1 of FIG. 6, when the shutter is released, the shutter blades are opened gradually and a film begins to be exposed after a time t.sub.2 that a pinhole is formed by the shutter blades. When an exposure finish time t.sub.3 determined in accordance with a film speed and the brightness of a subject to be photographed is reached, the shutter blades are suddenly closed and the exposure operation is finished.
The programming shutter as described above requires the extremely stable opening characteristic of the shutter blades in order to maintain the exposure to be accurate. As a manner of stabilizing the opening characteristic of the shutter blades, a mechanism including a servo control system or a governor is known heretofore.
a conventional manner using the servo control system is now described.
An aperture value of the shutter blades is detected by a light receiving element of which a light receiving area is varied in accordance with the opening amount of the shutter blades and is fed back to a servo amplifier. The servo amplifier is supplied as a command value with a voltage signal similar to the opening characteristic curve shown in FIG. 6. Since the servo amplifier drives a servomotor for driving the shutter blades in accordance with a difference between the command value and the aperture value, the aperture value of the shutter blades follows the command value.
However, such a servo control system requires the expensive servo amplifier and servomotor. Further, in the case of an apparatus like a camera using a power source such as a battery having low reliability, an extremely high-degree circuit technique is required to produce the command value similar to the required opening characteristic curve.
In addition, when the mechanism including the governor for stabilizing the opening characteristic is employed, the control operation thereof is of open type and the exposure accuracy depends on a mechanical accuracy. Accordingly, the mechanism is complicated in order to obtain the sufficient accuracy. Particularly, since the photographing position of a camera is not fixed to change to various positions such as vertical position, horizontal position, overhead position, elevation position and the like and the weight distribution of mechanical members is varied in accordance with the positional variation, the mechanism is complicated in order to obtain sufficient accuracy in any photographing position.